Symptoms of bad cat converter?

2000 Mazda Protege ES with 151k miles on it

What are the symptoms of a bad catalytic converter?

The CEL keeps coming on but the error code is "vague" and sounds to me like it could be the catcon or even O2 sensors!

Money is tight for me and I can't replace BOTH the O2 sensors and catcon at this time and want to replace only what is bad.

There are two O2 sensors total cost abt $200...and of course the catcon is abt $225.... not including labor for all this.

My CEL keeps coming on and my gas mileage is poor.

I always thought a catcon was good for life of the car. Is 151k too soon for catcon to be bad?

If you were gonna choose between the catcon or the two O2 sensors.... which one would you have replaced given my symptoms above?

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I'd agree that a CEL without knowing the code is quite vague! The lack of the code means that I'm making wild guesses, based upon very little information.

Poor gas mileage is a good indication that something is wrong. However, the current fuel blends deliver less mileage than they did without the added alcohol A bad front O2 sensor, or a clogged converter are just two of several problems that may cause your symptoms. It's unlikely that the rear O2 sensor is bad. While it might cause a CEL, it should not have anything to do with poor gas mileage. At 150K, you should have already replaced plugs, the fuel filter, and possibly the plug wires. The rubber timing belt is also past due for replacement. (If the 2000 Mazda Protege ES with 151k miles on it has a timing belt instead of a chain.) Converters may last 100,000 miles or more. You have 150,000, so it's also suspect. If it is partially clogged, this might be a cause of poor gas mileage.

Replacement Bosch universal 4wire O2 sensors are available for less than $100 ea. They come with a splice "kit", and reuse the original connectors.

I'd replace the fuel filter, plugs, and the front O2 sensor, and see what happens next.

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OK will do

The CEL came on again so this time I'm going to buy a cheap scanner and read it myself and will get back with a code

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You shouldn't need to do that. Auto parts stores will loan you a scanner. I used an Auto Zone scanner in their parking lot, read the code and reset the CEL. For me, it was a spurious 2nd O2 sensor code that never returned.

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the only real symptom i know of for a failed catalytic converter is failing to pass the emissions test ;) but seriously, here's a nice summary of the issue http://auto.howstuffworks.com/question482.htm if your car is running smoothly, performance is strong, and the millage hasn't changed, i would suspect a bad or loose gas cap is setting off the cel, but it really could be a number of things, most of which are emissions related.

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Trouble is that failing the EMC test by itself does not directly indicate just a bad converter. Even the code for "Poor Converter Efficiency" may be caused by something other than the converter. It's possible to replace the converter and still fail, or pass and have the ECU light come back on in few hundred miles. The key is to find out and fix whatever caused the converter to fail, then replace the bad converter.

Another part of the puzzle is that a brand new converter is more efficient than a used one in "normal" condition.

"the only real symptom i know of for a failed catalytic converter is failing to pass the emissions test "

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